Unity实现简单虚拟摇杆

最近一直在倒腾用ugui做虚拟摇杆，网上普遍的的做法就是使用以下的代码，但是这个有些注意事项，第一点就是canvas的render mode必须是screen space overlay，第二点就是挂载这个脚本的锚点的x,y必须是0.5，如图下：

using unityengine;
using unityengine.eventsystems;
public class joystick : monobehaviour, idraghandler, ienddraghandler
{
 transform point;
 vector3 startpos;//开始位置
 vector3 dir;//方向
 float radius = 0;//需要移动的半径
 void start()
 {
 point = transform.getchild(0);
 radius = (transform as recttransform).sizedelta.x * 0.5f;
 startpos = point.position;
 }
 public void ondrag(pointereventdata eventdata)
 {
 point.position = eventdata.position;
 dir = (point.position - startpos).normalized;
 if (vector3.sqrmagnitude(point.position - startpos) > radius * radius)
 point.position = startpos + dir * radius;
 }
 public void onenddrag(pointereventdata eventdata)
 {
 point.localposition = vector3.zero;
 }
}

如果canvas的render mode是screen space camera，这样的话上面的代码是不能满足要求的，花了一点时间才发现是这个原因，导致上面的代码不适用的，最后把代码重写了一下，终于可以成功了！

public class joystick : monobehaviour, idragevent
{
 private canvas canvas;
 private recttransform recttransform;//坐标
 private static quaternion amendangle;
 private static float mradius = 0,v=0, h=0;
 private static transform point;
 private static vector3 initpos;
 private static vector2 startpos;
 private void start()
 {
 point = transform.getchild(0);
 canvas = gameobject.find("uiroot").getcomponent<canvas>();
 recttransform = transform as recttransform; //也可以写成this.getcomponent<recttransform>(),但是不建议；
 mradius = (transform as recttransform).sizedelta.x * 0.5f;
 initpos = point.localposition;
 h = v = 0;
 }
 
 public void onbegindrag(pointereventdata eventdata)
 {
 recttransformutility.screenpointtolocalpointinrectangle(recttransform, eventdata.position, canvas.worldcamera, out startpos);
 startpos = eventdata.position - startpos;
 h = v = 0; 
 }
 
 public void ondrag(pointereventdata eventdata)
 {
 point.localposition = eventdata.position - startpos;
 vector3 dir = (point.localposition - initpos).normalized;
 v = dir.normalized.x; h = dir.normalized.y;
 if (vector3.sqrmagnitude(point.localposition - initpos) > mradius * mradius)
 point.localposition = initpos + dir * mradius;
 }
 
 public void onenddrag(pointereventdata eventdata)
 {
 point.localposition = vector3.zero;
 h = v = 0; 
 }
}

recttransformutility.screenpointtolocalpointinrectangle(recttransform, eventdata.position, canvas.worldcamera, out startpos)这个的startpos返回的是点击屏幕的坐标，recttransform是这个脚本挂载物体上的recttransform的组件，然后减去eventdata.position就知道坐标的偏移值了，看一下代码应该都可以了解意思，这里就不过多的解释了。

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